Magnetic field supported plasma generating devices are widely known and used. They are used especially as sputtering devices to coat substrates with thin films (PVD). Such plasma discharges are also used to remove material from substrates, which is also called sputter-etching. Further, such plasma discharges are also Used with so-called PECVD methods where, by means of plasma activation from the gas phase, a chemical deposition of thin films takes place on the substrates. Mixed PVD and CVD methods are also common. Most suitable and widely spread are plasma generating devices that function according to the so-called magnetron principle. Magnetron sputtering, for instance, has become a commonly used method due to the high sputtering rates that can be reached and the simple operation of the arrangement as well as the easy controllability. The magnetron sputtering source can be used in any position and its many ways of operation as a wide area source are advantageous for fully automated production equipment. Such a source consists of a cathode which is applied with a negative voltage in a vacuum chamber, where at the same time a plasma discharge is maintained by an inert gas such as argon or a mixture of inert gases with reactive gases, and the so produced ion bombardment sputters the target and makes it possible to deposit a thin film on a substrate positioned in the chamber. In order to reach a high plasma density and thus a high sputtering rate the plasma discharges are concentrated by means of magnetic fields. This, for example, is performed with magnetron sources by arranging magnetic field generating devices on, beside, or on the back side of the cathode or the target in such a way that the magnetic field enters and exits through the target so that a tunnel-like structure is formed which on the target front side acts as an electron trap for the plasma discharge. The effect is further enhanced when the tunnel-like electron trap is formed like a ring and thus forms a closed loop on the front side of the target. The electron trap thus becomes very efficient. Such an apparatus is described in the German Patent Specification DE 24 17 288.
The above described magnetron source has the disadvantages that the elongated form of the plasma loop leads to a trench-like erosion of the target material. This trench-like erosion leads to a very bad utilization of the target material in a range of 20 to 30%. According to the German Offenlegungsschrift DE-OS 27 07 144, which is important prior art related to the present invention, it is suggested to move the magnetic electron trap and the target relative to each other in order to neutralize this disadvantage. It is disclosed that not only the target can be moved above the magnet system as a rotating plate or as a rotating cylinder, for example, but also the magnetic system itself can be moved under the target. In both cases the target is eroded more homogeneously. The same result can be reached with a wandering electro-magnetic field. In both cases, it is suggested that the entire electron trap or the closed loop discharge be moved relative to the target. The practical realization of the electro-magnetic version is linked to high efforts and costs, for high magnetic flow densities of some hundred Gauss must be produced within a small space. The mechanical moving of the whole magnetic system as well as of the target is also very space consuming when practically realized and leads to large apparatus sizes. One example for a cylindrical target moving around a fixed magnetic system is shown in the U.S. Pat. No. 4,356,073. For the arrangement where the magnetic system is positioned inside the cylinder the presented solution requires high constructional efforts.